1. Field of the Invention
The present invention relates to a system for efficiently culturing algae such as Spirulina and a method of culturing the algae by using the system.
2. Disclosure of the Prior Art
Spirulina is a kind of blue-green algae living at salt lakes in the tropical regions. For example, it is well known that Spirulina lives in the Chad Lake of the Republic of Chad located at the Sahara Desert of Africa. Tribes living around the Chad Lake have ingested Spirulina as a protein source from time immemorial. The protein of Spirulina is composed of a lot of amino acids essential to human being, and richly contains minerals and nutrient substances except for vitamin C. Thus, Spirulina is drawing public attention as a nutrition-supplementary food for keeping human health. In addition, plans for using Spirulina as the protein source in space is in progress in National Aerospace Laboratory in Japan and NASA.
By the way, general crops are harvested once or twice a year. However, Spirulina can be harvested every time period of 5 to 20 days. When suitable conditions are selected, it is possible to harvest Spirulina 50 times or more a year. Therefore, Spirulina is expected as one means for solving the food problem of the earth. Additionally, since Spirilina has 6 to 8 times oxygen generating capability than general plants during photosynthesis, it has been used in CLESS (Controlled Life and Ecological Support System) on a space station project of NASA.
Spirulina has been usually cultured in various world places under a natural growth condition by the use of a culture pool, which has a relatively shallow depth for exposing a culture fluid containing algae to sunlight. For example, an upper-opened type pool having a width of about 10 m, a length of about 50 m and a depth of 20 to 25 cm is used as the culture pool. In addition, the pool has a separation board extending in the lengthwise direction at a substantially center of the width thereof to make a circulation way of the culture fluid in the pool. Spirulina can be cultured in the pool according to the following method. First, the pool is filled with the culture fluid containing Spirulina. Then, a circulation of the culture fluid along the circulation way in the pool is performed by stirrers, while sunlight being radiated to the culture fluid. In this time, carbon dioxide gas required for the photosynthesis of Spirulina is continuously supplied to the culture fluid by a pump. After the elapse of 5 to 20 days, Spirulina grows up to 300 .mu.m or more. The grown Spirulina is removed together with immature Spirulina from the culture fluid by a filtration.
By the way, only sunlight is used in this culturing method. Therefore, the growth rate of Spirulina lowers during the nighttime, or when it is cloudy day or rainy day. In addition, since the grown Spirulina floating in the culture fluid deteriorates the transmittance of sunlight into the culture fluid, a sufficient amount of sunlight required for the photosynthesis does not reach the immature Spirulina in the bottom of the culture pool. When a heavy rain or flood disaster happens, there is a possibility that most of the grown Spirulina is lost by an overflow of the culture fluid from the culture pool. Moreover, since the culture pool is usually designed to have a relatively small depth, there is a problem that carbon dioxide gas supplied from the bottom of the culture pool passes through the culture fluid without being sufficiently absorbed by the culture fluid. Since the above culturing method is based on a batch operation, the culturing of the immature Spirulina must be stopped every harvest time of the grown Spirulina.